1. Field of the Invention
This invention relates to an ink composition, in particular, an ink composition for inkjet recording which is excellent in the image durability in storage and, among all, fastness to gas and penetration into paper.
2. Description of the Related Art
With the recent diffusion of computers, inkjet printers have been widely employed in printing on papers, films, fabrics and so on not only in offices but also in homes.
Inkjet recording methods include a system of jetting ink droplets under pressurization with the use of a piezo device, a system of foaming an ink by heating and thus jetting ink droplets, a system using ultrasonic wave, and a system of electrostatically sucking and jetting ink droplets. As ink compositions for inkjet recording by these systems, use is made of water-base inks, oil-base inks or solid (molten) inks. Among these inks, water-base inks are mainly employed from the viewpoints of production, handling properties, odor, safety and so on.
Requirements for a coloring agent to be used in these inkjet recording inks are as follows: being highly soluble in solvents, enabling high density recording, having a favorable color hue, having high fastness to light, heat, air, water and chemicals, having favorable fixation properties on an image receiving material with little bleeding, being excellent in storage properties as an ink, having no toxicity, having a high purity, and being available at a low cost. However, it is highly difficult to search for a coloring agent satisfying these requirements at a high level.
Although various dyes and pigments have been already proposed and practically employed in inkjet recording, no coloring agent satisfying all of the above requirements has been found out yet. Using well-known dyes and pigments typified by those having color indexes (C.I.) assigned thereto, it is highly difficult to satisfy both of such color hue and fastness as required in inkjet recording inks.
When a recorded image is exposed to the atmosphere over a long period of time, in particular, a cyan ink is seriously deteriorated and the image becomes reddish in some cases. It appears that the cyan ink deterioration is caused by various oxidative gases typified by ozone contained in the atmosphere.
As examples of a method for improving the fastness to gas (fastness to ozone), JP-A-2002-294117, JP-A-2002-356639, JP-A-2003-138184 and JP-A-2003-138185 disclose methods of improving the fastness to gas (fastness to ozone) by adding a specific compound. However, the thus achieved the fastness to gas (fastness to ozone) is still insufficient.